All percentages herein are by weight.
This invention relates generally to chemical reactor apparatus and more specifically to the manufacture of metallic grids used to support a catalyst during production of nitric acid in high temperature reactor vessels.
In the manufacture of nitric acid by the oxidation of ammonia, high velocity gas containing about 10% by volume ammonia is passed through a platinum catalyst bed and reacted at high temperatures near 1700.degree. F. to form acid.
The catalyst bed is typically formed by several layers of platinum wire gauze (e.g. 20 layers of No. 8 mesh screen) supported by a grid of larger bars or plates. The grid must support the weight of the catalyst (about 100 ounces per square foot) and resist the high aerodynamic drag during several months of continuous operation at high temperatures. Further, the grid material must resist oxidation, carburization, and nitriding by the hot flue gas while remaining straight and flat so as to prevent unoxidized ammonia from by-passing the catalyst.
There is a large demand for chemical reactor apparatus of this type since over 8,000 tons of nitric acid were produced by this method in the United States during 1985 and the use is increasing each year.
Typically, these support grids have been made from nickel-base superalloys which last about three years before replacement. Each grid uses about 3000 pounds of the expensive superalloy material and over 100 units are replaced each year on the average.
Thus, it should be apparent that an improved, longer lasting support grid is needed by the industry.
In addition to the aforementioned required corrosion resistance, it would be desirable for the support grids to be easily weldable and formable both during initial fabrication and later, after use, so that repairs may be made as necessary. Thus, thermal stability is an important property for the grid material. Many nickel-base superalloys do not have good thermal stability and become brittle when exposed to high temperatures for long periods of time.
Another property very desirable in these support grids is high creep strength so that the structures do not distort, or at least distortion is held to a minimum during their long exposure to high temperature stress.
Some of the problems and complexities involved and additional background information related to the general field of this invention may be found in U.S. Pat. No. 4,438,084 and the art cited therein.